1. Field of the Invention
This invention relates to water-soluble herbicidal compositions in dry powdered form which include herbicidally active substituted phenoxy and/or benzoic acids that alone are not readily soluble in water.
2. Description of the Prior Art
Those herbicidal agents registered for use by commercial applicators are generally sold in concentrated form for economy of transport, and then diluted by the applicator either at a central distribution center, or less frequently at the point of use. The majority of effective herbicides and plant growth regulators are foliarly absorbed and therefore, to be effective, must be applied to the foliage of the target pest species. Other herbicides are root absorbed and the product must be applied in a manner as to be available to the roots of the target. This is commonly accomplished by spraying a dilute water solution, or dispersion of the desired pesticide on the vegetation to be treated. Most herbicides are therefore marketed as either (1) liquid or dry water-soluble formulations, (2) liquid, water emulsifiable formulations, or (3) solid or liquid water dispersible formulations. The concentrated formulations are diluted to the required effective concentration by the person doing the spray application. Thus, in order to obtain optimum effectiveness and to minimize agitation and other mechanical suspension requirements, water-soluble formulations are normally preferred.
Because of the difficulties of manufacturing a dry, soluble form of herbicide, most dry formulations are simply dispersible forms of essentially insoluble active ingredients. Typical examples of formulations are (1) wettable powders, (2) water dispersible granules, or (3) dry flowables. Formulations of these types depend heavily on surfactants and grinding techniques to provide a dry formulation of active ingredients that can be temporarily dispersed or suspended in water for spray application. Even when a dispersion can initially be obtained in water, the time of full dispersion is usually limited, thus requiring stirring, agitation with air, or other mechanical mixing. Dispersions of this type present additional problems in that the material tends to clog spray nozzles and other distribution components, and require the user to prepare smaller than desired batches in order to minimize application problems.
Because of the problems associated with attempting to prepare the dry powdered herbicide, suppliers have resorted in some instances to dissolution of the active ingredient in an organic solvent such as mineral spirits or the like. The concentrated formulation, containing suitable surfactants, is then diluted with water to form a dispersion that again usually necessitates some type of agitation to maintain the phases substantially homogeneous for a useful period of time.
In instances where the herbicides are dissolved in a solvent for shipment as a concentrate, the solvent presents health and physical hazards to the manufacturer as well as the user, the solvents add to the overall cost of the product, and the solvent agent is oftentimes phytotoxic to desirable plant species.
The wettable powders and solvent dissolved herbicides are frequently packaged in plastic containers and disposal of these plastic packages is becoming increasingly difficult from an environmental standpoint.
Substituted phenoxy and/or benzoic acid herbicides such as (2,4-dichlorophenoxy)acetic acid [2,4-D], 4-(2,4-dichlorophenoxy)butanoic acid [2,4-DB], (.+-.)-2-(4-chloro-2-methylphenoxy)propanoic acid [MCPP], (4-chloro-2-methylphenoxy)acetic acid [MCPA], (.+-.)-2-(2,4-dichlorophenoxy)propanoic acid [dichlorprop], 3,6-dichloro-2-methoxybenzoic acid [dicamba], and 3-amino-2,5-dichlorobenzoic acid have long been used to control unwanted vegetation.
These substituted phenoxy and/or benzoic acid herbicides are white crystalline solids with very low vapor pressures and low water solubilities. They are soluble only in alkaline solutions or polar organic solvents.
Phenoxy and/or benzoic acid herbicides are available commercially as acid, ester, alkali metal, and amine salt formulations that can also be applied as mixtures with other herbicides. The alkali metal and especially the amine salt formulations are preferred because they are the most water-soluble and can be more readily applied as aqueous sprays. However, the esters must be applied either as emulsions in water, or as solutions in organic solvents such as oils. 2,4-D for example, is an insoluble crystalline material having a pK.sub.a of approximately 2.6. For ease of application, 2,4-D is normally converted to a water-soluble amine or mineral salt by the manufacturer and then dissolved by the applicator in a water carrier before use.
However, water-soluble substituted phenoxy and/or benzoic acid salts exhibiting herbicidal activity are difficult to prepare in a dry state. Soluble salts such as potassium or sodium or dimethylamine must be first prepared in water or a solvent and then the solvent removed. This requires special equipment, is energy intensive, and frequently generates undesirable waste products. As a consequence, most dry forms of herbicide that are marketed are not of a soluble type but rather are merely dispersible forms of the essentially insoluble herbicide acid which are distributed as a wettable powder or a wettable, dispersible granule. Although both inorganic and organic salt forms are commercially available, the most common salt form is the dimethylamine salt of the substituted phenoxy or substituted benzoic acid herbicide. Typical formulations range from about 20% to 50% active ingredient concentrations in water or solvent solutions.